The present invention relates generally to overload relays for electromagnetic contactors, and more particularly to a control system for controlling the coil in an electromagnetic motor starter that uses an 8-pin, 8-bit microcontroller and I/O reduction circuitry to process at least 13 I/O""s and calculate the operating temperature of a motor on a real-time basis.
In typical electromagnetic contactors, an overload relay is used to protect a particular load, such as a motor, from excess current and heat. Known overload relays incorporate bi-metal switches which require hand bending and tweaking to calibrate for a particular device. Needless to say, accuracy is suspect after field adjustments are made, such as once the full load amperage (FLA) adjustment is made. Further, such bi-metal switches and heaters used in existing overload relays, are typically connected in series with the contacts of a contactor. These bi-metal devices therefore require separate invasive connections between the contactor and overload relay, and not only increase the cost and size of the starter, but also the manufacturability of the device.
Another problem with such known electromagnetic overload relays is the use of current measurement devices such as current transformers and torroids. Current transformers are large and bulky, thereby adding to the overall size and cost of the device. Torroids are not as large, but still add to the size and expense of the package, and offer limited accuracy and range.
Therefore, it would be desirable to have a motor overload coil control capable of using the relatively tiny Hall effect sensors which have a much wider dynamic range for sensing current. Further, it would be desirable to have such a device that eliminates the need for the mechanical bi-metal switches.
The present invention provides a motor overload coil control that solves the aforementioned problems, while maintaining a relatively low cost control.
In accordance with one aspect of the invention, a motor overload coil control includes a number of inputs and outputs (I/O""s) to and from the motor overload coil control having j inputs and k outputs. The I/O""s include at least one reset input, a current sense input, a full load amp (FLA) input, a coil control input, a coil control output, and a status output. The control includes a microprocessor having n input lines, where n is less than j+k. A motor overload reduction circuit is provided for connecting the j inputs and the k outputs to the microcontroller in a manner to prioritize and consolidate X I/O""s such that the number of I/O""s is reduced, but still exceeds the number of I/O lines of the microcontroller. The microcontroller is programmed to use a number of its I/O pins to function as both an input and an output.
In accordance with another aspect of the invention, the microcontroller is programmed to calculate a first order differential equation to keep real-time track of the motor""s temperature and provide a status indication of that temperature when it exceeds a pre-determined range. The system can be set to activate an alarm and disable the motor at the expense of the process, or activate an alarm and a visual status indication, and maintain motor operation in critical processes.
The control includes an 8-bit, 8-pin microcontroller that together with an I/O reduction circuit, is capable of functioning 13 I/O""s with only 5 I/O pins on the microcontroller. Three remote inputs are received into the reduction circuit from a remote source. The remote signals are prioritized and reduced on one input line to the microcontroller. The control includes three-phase current measurement from three Hall effect current sensors, each positioned on a bus bar for each phase. The system includes amplification, half-wave rectification, and integration of the current signals to sum the current signals and provide a single input to the microcontroller. The system includes a full load amperage (FLA) adjustment to allow use on a wide range of motors. A status indicator is provided having an LED output for showing the status of the system and indicating the temperature of the motor, when appropriate. The system uses 24VDC coils, and after the coils are initially pulled in, the microcontroller generates a PWM signal to keep the appropriate coil pulled in at reduced power levels. An internal 24VDC sensor is used to ensure the proper level of DC voltage and to adjust the PWM for variances in the DC voltage level. A pushbutton test switch is provided, as well as a pushbutton local reset and a solid state remote reset. The use of an 8-bit, 8-pin microcontroller in such a motor overload coil control, allows the system to solve a first-order differential equation to keep real-time track of the operating temperature of the motor, while at the same time keeping the overall cost of the control relatively low. The microcontroller has a look-up table for finding an appropriate I2t value based on the FLA adjustment and the value of the sensed current. The I2t value is used in real-time monitoring of the operating temperature of the motor.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.